Polymeric multilayer membranes are well known in the art. The purposes, construction and properties of the layers in many of these membranes are often different from each other. In many cases one of the layers of such membranes is a conventional membrane, either a microporous membrane or a skinned (or partially skinned) asymmetric membrane.
U.S. Pat. No. 4,230,463 describes a two layer membrane in which the surface of a membrane is coated with an "occluding" layer of a second relatively nonseparating polymer that in essence "seals" one side of the membrane. This construction is used to reduce the contribution of nonselective flow through defects in the skin and results in a membrane whose selectivity is intermediate between that of the polymer making up the skin and that used in the coating. Thus the separation achieved is less than that expected for a defect-free skin, but sufficiently high to be useful.
U.S. Pat. No. 4,813,983 describes a gas separation membrane having a relatively thick but porous polysulfone layer, and a thin, &lt;5 .mu.m, coating of a styrenic polymer. The membranes made in this patent do not have an overall selectivity that is higher than that expected for a defect-free polysulfone layer. The thin layers are formed by evaporation of the solvent of a solution of the styrenic polymer that is on the surface of the polysulfone.
U.S. Pat. No. 4,493,714 describes a gas separation membrane in which a porous polysulfone membrane is coated with a thin layer of a silicon containing polyurea. The silicon containing polyurea is coated onto the polysulfone by an "interfacial" polymerization carried out on the surface of the polysulfone. The thickness of the polyurea layer is about 100 to 10,000 A, but the overall selectivity of the membrane is less than that calculated for the polysulfone layer, assuming that layer was defect-free.
U.S. Pat. No. 4,631,075 describes a gas separating membrane having three layers, a relatively thick, porous, supporting layer, a polyorganosiloxane layer about 300 A to about 5 .mu.m thick, and a thin layer, 100 to 5,000 A thick, of specific composition having a certain separation properties. The present invention does not require three layers and a polyorganosiloxane.
U.S. Pat. No. 4,767,422 describes the "post-treatment" of composite membranes to repair defects in a relatively thin separation layer, by contacting the membranes with a volatile liquid that may optionally contain minute amounts of nonvolatile materials, such as polymers. The patent states that defect pores in the membrane are plugged but no mention is made of forming any additional polymer layers on the membrane.
The present invention provides a gas separation membrane, and a process for making such a membrane, said membrane comprising a relatively thick polymeric layer coated with an ultrathin layer of a polymer of relatively high selectivity, said membrane having a selectivity equal to or greater than that calculated for the thick layer, assuming said thick layer to be defect-free. The advantage of using an ultrathin layer coated on a thick layer is that good selectivity can be attained while maintaining a high flux of the gases to be separated. In order to achieve the requisite selectivity, it is necessary that the ultrathin layer be relatively defect-free, a condition not achieved in membranes of the prior art.
Accordingly, it is an object of the present invention to provide a gas separation membrane of high selectivity.
It is a further object of the present invention to provide a gas separation membrane that allows a high flux of the gas to be separated.
It is an additional object to provide a novel process for making selective gas separation membranes.